"To err is human; to forgive, divine." Castiel does not so much as blink after he reads this. An angel's mind is a heavenly supercomputer, capable of processing at the speed of light, and the amount of information he reviews before formulating an opinion for Dean is, in human terms, astronomical. Which means that his answer is honest. "But if the divine are capable of error, and humans can forgive, then we are not so different."
"It's poetry, Cas," Dean says, in the tone that strongly suggests he doesn't care. But Castiel cares, because it's the little things like human literature that open his eyes to the reality of things, and that is beautiful; it is profound, because in a universe where even the most carefully laid plans and schemes and purposes go awry, art never fails to point the way to truth. Creation is art. God has not forgotten us, Castiel thinks.